Ultra high frequency transmitter



Sept. 26, 1939. o. E. Dow Er AL ULTRA HIGH FREQUENCY TRANSMITTER Filed May 6, 1936 QEWRWY rubs.

Y Patented Sept. `26, 1939 UNITED STATI-:s

PATENT 'GFFICE 2,174,154 ULTRA HIGH FREQUENCY TRANSMITTER orviue E. now and Nils E. Lindenblad, Port Jefferson, N. Y., assignors to Radio Corporation of American a corporation of Delaware application May c, 1936, serial No. 78,124

2 Claims.

l0 oscillator controlled by a concentric line resonator, and, coupled to the output of the oscillator, .a push-pull power amplifier to whose grids are coupled the amplified video frequencies of an extremely wide band. The oscillator is arranged ,l5 `to generateoscillations whosefrequencies are of the order of 177` megacycles corresponding to 1.695 meters. In the output of the power ampliiler and coupled to it through a novel type of .Lecher wire transformer is a monitor circuit for v20 checking the operation of the transmitter.

' Among the features ofthe invention is the tuned concentric line resonator whose frequency is variable by changing at will the capacity between .the conductors.

A complete description of the invention follows, accompanied by a] drawing wherein vthe single figure schematically shows a complete transmitting system including master oscillator, power amplier, modulator and monitor circuits.

Referring to thedrawing in more detail, there is shown a master oscillator circuit comprisinga pair of electron discharge devices I, 2 arranged in push-pull relation. There is provided a lament source of supply 3 connected in common 35 ,to the filaments of both electron 'discharge device oscillators. Each filament has in series with each of its legs an inductance 4 which is of such value that, in cooperation with the tuning condenser 5 vand" series connected by-pass condensers 6 for n radio frequencies, proper. regeneration is obtainedin the oscillator for best eiliciency. The equivalent inductive reactance of 4 is variable by the g condenser 5 in shunt with it.v Alament return -bias resistor 1 is shown connected between one 45 leg of the mament and ground. Connected to the grids of the oscillator devices I 2 is a tuned concentric tubular conductor. resonator line circuit comprising an inner conductor I and an outer conductor I0 for controlling 5o the frequency of the oscillations generated by the master oscillator. This frequency control resonator is generally ofthe type described in the article by Clarence W. Hansell, published in the A. I. E. E. August, 1935, Pages 852-857, except for 55 certain features. which will appear more fully (c1. 17e-171.5)l

hereinafter. .The grids of the electron discharge devices I, 2 are inductively coupled to the inner conductor 9 of the line by means of two singleturn coils placed radially in the space between the inner and outer conductors and so connected as to impress radio frequency voltages of opposite phase on the grids of the vacuum tubes. In order to reduce the inductive reactance of the grid circuit of both vacuum tubes, so as to keep the regeneration of proper pha'se, there is provided a parallel shunt circuit 8. Inner conductor 9 of the tuned concentric line resonator is, in practice,

made tohave a length slightly less than 'onequarter of the length of the operating wave, and

the capacity between the top II of the resonator l 'and the top of the inner line 9 varied'to give the desired frequency. The inner conductor comprises as a part thereof a pair of plates I2, i 3 which are connected together conductively by a screw and a phosphor bronze spring I5. In one L position, both plates I2 and I3 may be madeto appear like a single plate, in which case there will be minimum capacity between the inner conductor 9 and the upper part I I of outer conductor I 0 with a cdnsequentmaximum frequency ofl oscillation in .the resonator. At the other extreme, both plates maybe made to assume a maximum area by turning screw I6 connected to shaft I4, in which case there is now a maximum capacity between the inner'conductor 9 and plate Il with-a consequent minimum frequency of oscillations in the resonator.

The output oi' the master oscillator comprises a parallel tuned circuit consisting of a variable condenser I1 and an inductor I8, I8', the inductance of which has substantially uniformly distributed constants. This-inductor is composed of two copper rods I8 and I8 of equal length and enclosed by concentric copper sleeves ISand I9 whichare grounded. One conductor I8 is connected to the anode of tube z while conductor I8 connected to the anode of ltube I. The-opposite ends of I8 and I8 are connected together by any suitable conductor M so positioned over ground sheet 2l) as to make its capacity per unit lengththe same as the concentric conductors` I 9-I8 and I9'I8'. The conductor 44 must be long enough to provide sumcient coupling tothe power amplifier grid circuit. In view of the physical configuration of inductance I8, I8', whichincludes two' parallel connecte/drportions and a straight portion Il, it will herein bereferred to as a Lecher wire output. The length of this Lecher wire output, as measured from the anodes as short as possible.

point of portion 44 of the Lecher wire system, is made to be about one-eighth of a wavelength in order to prevent reaction upon the master oscillator device due to modulation of the power ampliner. The load on the master oscillator. which in this case is the power amplier grid circuit, may be replaced by an equivalent series resistance (variable over the modulation cycle) in series with the Lecher wire output I8, I8' of the two master oscillator electron discharge devices. The inductive reactance of the output I8, I8' at the operating frequency is fairly low, of the order of 100 ohms, and hence the equivalent series load' resistance may be made small,r

about two ohms. Consequently, any variation of the series load resistance of two ohms due to modulation will have little effect on the master oscillator anode inductive reactance of 100 ohms.

Coupled to the straight portion 4I of the Lecher wire output I8, I8 are a pair of power amplifier electron discharge devices 2l, 22 arranged in push pull relation to each other. The filament circuits of the tubes 2|, 22 are heated by a common source of energy 23 which feeds the laments through radio frequency choke coils 23. Condensers 25 serve to by-pass the radio frequency energy in the lament connections to ground. A pair of neutralizing condensers 21 cross-connect the grids of the devices 2l, 22 to Vthe anodes of the associated devices in the conventional manner. The electron discharge devices, 2l,- 22, are capable of oscillating at frequencies as high as 400 megacycles per second, hence it is necessary to'make the leads from the grid and anodes to the neutralizing condensers If these leads are made even moderately long, the relatively low in- ,ductance of the leads will have an appreciable inductive reactance at frequencies of the order of 300 megacycles. This inductance may be sumciently high to allow the tubes to oscillate at some'very high frequency. The ideal condition is to have zero reactance between the grid and neutralizing condenser and between the plate and neutralizing condenser.

The center tap of the power amplifier grid circuitvis connected through a resistance 4I to the anodes of the modulator tubes 4I which are maintained at a positive potential of about 250 volts. However, the filaments of the, power amplifier devices 2 I, 22 are maintained at a positive potential of 500 volts relative to ground, and'consequently it can be said that the grids of the power amplifiers are at an equivalent negative potential of 250 volts relative to the nlaments thereof. 'Ihis arrangement of connecting the anodes of the modulation tubes conductively to the power ampliiier grids and increasing the power amplifier filament return bias by 250 volts simplifies the design of the circuit greatly. f

The output circuit of the power amplier comprises a symmetrically bent Lecher wire system comprising two halves123, 23' connected to which, in parallel, is a variable condenser 2l for tuning the output. The two halves 22, 23 are provided merely to give symmetry` to the physical ccniglguration of the circuit, so that in case there is any influence on one half of the tuned circuit by the adjacent lament of internal tube elements, there will also be a similar effect on the other half of the Lecher wire output. Of course, it will be lmderstood that, if desired, one-half of the Lecher wire output for the power amplifier can be dispensed with.

Coupled to the power amplifier output through leads 3l there are shown a pair of terminals wmcn connect with any suitable utilization circuit such as an antenna. It will be observed that leads 3l are inductively connected to the Lecher wire system 28, 29 by means of a loop', the center point of which is grounded at 3i. This ground connection serves to prevent push-push voltages on the antenna system. Also coupled to leads 3l is a step-down transformer comprising a U-shaped loop 32 whose legs are preferably each a quarter wavelength long, as indicated, although the length of each leg can be any odd number of quarter wavelengths long. This loop is the equivalent of a step-down transformer and serves to step-down the impedance at the antenna terminals from a high value to the low value at the lower end of the loop, which is shown inductively coupled to a diode 33. This U-shaped step-down transformer, it will be observed, is also grounded at the center point 34, to pevent push-push oscillations. Loop 32 is merely a coupling link between the output of the power amplifier and a monitor circuit comprising diode 33 whose filament is closely coupled inductively to the step-down transformer 32. In the output .circuit of diode 33, which in practice may comprise an RCA type 955 tube, is a filter circuit 35, here shown conventionally, in turn, coupled to a voltmeter 3l. Circuit 35 serves to-illterA out the carrier frequency from the voltmeter. The purpose of the monitoring 'circuit is to measure the amplitude of the carrier from the power amplifiers, and also to measure the `modulation on the carrier.

Iiil

Should be noted that the quarter wavelength long serves to amplify the video frequency input from the value of .35 volt rms., as supplied from thc g studio to the flrst amplifier, to about 80 volts rms.

as effective in modulating the grids of the electron discharge devices 2|, 22. For this purpose there isqprovided a nrst pentode low power tube 33 which in practice may be an RCA 6C6, which drives a second pentode -tube 39 whose output is,

in turn.' connected to the grids of two other pentode tubes Il in parallel. From the ouput of pentode tubes 3l is a connection to the grids of the power amplifier. In series with this connection is a resistor 4I to prevent push-push oscillations in the power amplifier. This resistor acts as a large series reactance to voltages of the carrier frequency and has small stray capacitance for voltages of thevideo frequencies, and damps high frequency push-push parasitic Electron discharge devices 30 and I, may, in practice, comprise RCA type 802-. tubes. Allv pentode tubes are of the well-known type which include bides the anode and cathode, a control grid and a suppressorq grid connected in known fashion. Coupled to the anodes of the two parand an -inductance which serves to compensate for the tube and circuit output capacity. 'Ihe video frequencies supplied from the studio may have a very wire range extending from, let us say, 20 cycles to 1,500,000 cycles.

In the operation of the transmitter circuit, the voltage impulsa from the master oscillator, which have a frequency of the order of 177 megacycles per second, are impressed von the power oscillations.

amplifier grids in push-pull relation, and ampli- Y l 9,174,154 fied through amplifiers u, u. 'me degree or ampliiication is' varied by varying the bias ofthe grids of 2|, 22 in `parallel in accordance with tude modulated signal is sent out over the air through the antenna circuit coupled to the Lecher wire output o i the power ampliner. The monitor circuit 33, di, serves to measure the amplitude and degree of modulation of the carrier.

Although the invention has been described herelnabovewith reference to a particular embodiment, it will be understood, of course', that the scope thereof is not limited thereto since the various features thereof arecapable of being used in other circuit arrangements and in moditied forms without departing from the spirit of the present invention.

What is claimed is:

1. A pair of electron discharge' devices connected in push-pull relationship, each 'of said devices having an anode, cathode and a control electrode, connections between said devices coupling corresponding electrodes together, a resistive connection from the electrical center of said connection coupling said control electrodes together to the positive terminal of a source of potential, whereby said control electrodes are v maintained at a positive potential relative to a point of xed radio frequency potential, meansl for maintaining said cathodes at a higher posi- A tive potential relative to said point of fixed radio frequency potential, whereby said control electrodes are at a negative potential relative to said cathodes, and a modulator circuit coupled to said control electrodes, said resistive connection being in the circuit coupling said modulator to said control electrodes and functioning to damp high -frequency push-push parasitic oscillations.

2. A pair of electron-discharge device amplitiers connected in push-pull relationship, each of said devices having an anode, a cathode, and a control electrode; connections between said devices coupling corresponding electrodes together; a circuit from the electrical center of that connection which couples said control electrodes together to the positive terminal of a source of potential. whereby, said control electrodes are maintained at a positive potential relative to a point of iixed radio frequency potential; said circuit incuding -means for supplying modulating potentials to saidampliers and including a resistance between said means and the electrical center of said last connection; and means for maintaining 'said cathodes at a4 higher positive potential relative to said point of xed radio frequency potential, whereby said control electrodes are at a negative potential relative to said cathodes.

ORVILLE E. DOW. NILS E. LINDENBLAD. 

